Why? I am so confused.

I am looking for a good Mordekaiser build but keep finding guides that have him getting Magic Penetration and Ability Power.

Because Magic Penetration and Ability Power actually allow him to hurt things, which charges up his shield faster. That said, it's nowhere near as tanky as a tank built Mord.

There is no up to date guide that I know of written for Mordekaiser because to put it quite plainly, nobody with good enough recent ranked credentials can vouch for their build because Mordekaiser is hardly ever used after his massive nerf.

I would suggest, if you haven't purchased this champion yet, to steer clear of him for now. He is in my opinion the bottom of the barrel right next to Cassiopeia. If you insist however, this build worked for me some time ago:

Sorcerer's Boots
Randuin's Omen
Force of Nature
Rylai's Crystal Scepter
Abyssal Scepter
Zhonyas Ring(I guess you replace it with one of the new two, I suggest Rabadon's)

Now, this build gave you incredible strength if you were allowed to free farm a lane. My highest ranked kills was 33 and I nearly carried a 4v5 with it. Force of nature is a controversial item for Mordekaiser but the mobility, the MR, and additional regeneration are too good to pass up. You see unlike other champions, you need not to look at cost efficiency, but how much you are getting per item slot, because of your immense farming abilities. Force of Nature is the most MR for your slot and gives you much needed mobility.

But once again, do not purchase him, it is a lot of IP to waste for one huge dissapointment.

his early game is no longer soo strong that he can stand there and farm all day, but his later game was buffed, people now say hes useless which shows they never knew how to play him properly before.

There Actually is a High Elo build, has the highest success rate ive seen his mina items are Frozemmallet and rylais. but i still preffer spell vampmorde

Mordekaiser is a pubstomp champ, but if you still want to play him, build FoN/Randuin's with AP after that. If you want to become a good player, learn a tank that contributes more to your team.

rickless, it is unkind of you to understate these nerfs to Mordekaiser. 3150 IP is quite a lot of games and if you knew anything about Mordekaiser you would know how drastic these changes were. 5% decay versus 3%, this is almost TWICE the rate of shield decay.

Now, one nerf that many of you may have dismissed is this:

Siphon of Destruction
Damage reduced to 65/105/145/185/225 from 85/110/155/200/245
Ability power ratio increased to 0.6 from 0.4
Base shield generation reduced to 1/2/3/4/5 from 6/9/12/15/18

Before Patch:
Level 1 - 36 Bonus shield
Level 2 - 54 Bonus shield
Level 3 - 108 Bonus Shield
Level 4 - 135 Bonus Shield
Level 5 - 162 Bonus Shield

After Patch:
Level 1 - 6 Bonus Shield
Level 2 - 12 Bonus Shield
Level 3 - 18 Bonus Shield
Level 4 - 36 Bonus Shield
Level 5 - 40 Bonus Shield

This nerf was the most devastating blow Mordekaiser has had yet, of course these numbers are assuming you line up all six minions for each Siphon.

Oh and the ghost was not nerfed a 'bit' it was a MAJOR nerf. Before, the ghost was nearly invincible and lasted the entire duration. Now any smart team focusfires the ghost instead of Mordekaiser and it falls quickly. I also call the lack of Spirit Visage's HP regen a nerf as you can no longer rush it. Randuins Omen is indeed, another nerf and the AP ratio on the mace of spades is hardly worth mentioning.

I apologize if my tone is offensive, I didn't mean so I am just a formerly dedicated Mordekaiser player for nearly four months and watched my champion become fotm and then systematically destroyed. There is 'some' hope for Mordekaiser but it does not lie in Siphon of Destruction.

With your shield decaying at such an obscenely high rate and your health falling like mad desperately trying to maintain your shield with siphon it's just complete nonsense now. If you do choose to play Mordekaiser, use his W, and max it out first.

W gives you the most shield for your health and you can cast it on your own minions to 'last hit' from afar or to give you a bit of a buffer zone in terms of shield before you approach your enemies harass range. But really, Mordekaiser is the worst champion in the game besides Cassiopeia. That is just my opinion and your welcome to argue it. He has no CC, and without his lane domination niche and his powerful ghost it's just not worth it.

While I don't play him as much as I used to, the only nerf I was upset with is the shield decay rate. Everything else I'm content with. He is not as untouchable in the laning phase any more, his W is STILL underrated and underused... I STILL see Mordekaisers rush FoN. He takes a little finesse to work now.

It's also a little more difficult to get CDR on him, but I tried something a little different:
Randuin's Omen (7%)
Boots of Lucidity (15%)
Abyssal Scepter
Rylai's Crystal Scepter
Lichbane
...lots of wards, and rolling Elixir of Brilliance (10%).

32% CDR + masteries and runes put me at max.

While it wasn't my most impressive game, I did well, and still feel confident that Mordekaiser is indeed viable.

But he's NOT a tank. Stop @%#ing playing him, and building him, like he's a tank. He's a soak, at best, and he soaks the most damage by dealing as much damage as possible.

OP wanted a good AP Morde build, not to be told why he's useless. What's worked well for me as DPS/Off-Tank Morde is the following:

Boots of Swiftness - Key because he's a slow piece of metal with no slow/stun/disable. Although I would argue that the new CDR boots would be better, haven't tried them out yet though.
Rylai's Crystal Scepter - Gives him massive AOE slows, and you can use your Ult to DOT someone and keep up with them ensuring they DIE and you get the ghost.
Abyssal Scepter - Seems kinda silly getting 2 scepters, but then again, the MR gained and MR lost by the opponents makes you deal actual damage with your abilities and helps your teams casters.
Deathcap - You could get this first, but I find that having AOE slows helps me get more kills than the additional AP would early, and when you complete it you get a huge Spike in damage.
Sunfire Cape - Your last tanky type item, with constant AOE slows, and huge shield tanking powers, you now have enough armor/MR and HP to just walk into battles and AOE the snot out of the team, not dying and dealing a lot of damage via Sunfire/Creeping Death/Ult/Siphon/Mace of Spades.

While most builds tell you to focus on Armor/MR instead of HP, and this is usually true with Morde, I find that when going AP, all the items you want anyway have these built in. So you can get the best of all the worlds. You get a decently high AP, you get surviveability with increasing your shield generation by reducing their MR, and increasing your AP. You get higher DPS output with AP than you do by simply being able to stand around and fire off another weak spell. You cause massive chaos when slowing the entire enemy team with Mace/Siphon and Creeping Death just chasing people with Sunfire while they take Creeping Death dmg, and being slowed.

I like this build because I feel it's more the feel of Mordekaiser. I would think that a champion like him would walk around the battlefield and that no one would want to be near him, always trying to run away while he tanks and deals large sums of AOE dmg.